1. Field of the Invention
The present invention relates to a component of glass-like carbon for CVD (Chemical Vapor Deposition) apparatus. Although this specification deals mainly with the inner tube for CVD apparatus, the present invention also covers various components to be arranged inside of CVD apparatus.
2. Description of the Prior Art
Production of semiconductor devices generally relies on so-called CVD process which involves gas-phase chemical reactions of reactant gases to deposit silicon, silicon nitride, etc. in the form of thin film on silicon wafers. In this process, silicon wafers are placed in an inner tube (shown in FIG. 1) for their uniform heating and controlled flow of reactant gases.
An inner tube for CVD apparatus is required to have good durability (heat resistance at 500° C. or above and corrosion resistance to reactant gases) under ordinary CVD conditions. It is also required to give off as little dust and impurity gas as possible. These requirements have conventionally been met by an inner tube made of quartz.
CVD process is based on the principle that heated reactant gases (as raw materials) decompose or react with one another to form film (such as polysilicon film and silicon nitride film) on silicon wafers. During CVD process, decomposition or reaction products of reactant gases deposit on the surface of the inner tube. The inner tube carrying such deposits (polysilicon, silicon nitride, etc.) is used repeatedly without replacement to maintain productivity. After continued use, the inner tube becomes covered with thick deposits, which eventually peel off from the inner tube to give fine particles. Such fine particles stick to silicon wafers, thereby reducing yields. Incidentally, “particles” denote particulate defects which are detected when wafers are examined by an optical tester.
One way to eliminate particles originating from deposit that peels off from the inner tube is to periodically demount and clean the inner tube to remove CVD deposit. Cleaning employs chemical solutions like hydrofluoric acid and nitric acid.
Such cleaning operation, however, reduces productivity and increases production cost. Therefore, the inner tube should permit CVD deposit to firmly adhere to it so that accumulated CVD deposit will not give rise to particles. Adhesion of CVD deposit to the inner tube is affected by the dimensional change of the inner tube which occurs when the CVD chamber is cooled and heated to unload and load wafers, respectively. The inner tube should ensure good adhesion despite its dimensional change, due to such cooling/heating cycles. Moreover, since cleaning procedure to remove CVD deposit is unavoidable, it is essential for the inner tube to have good corrosion resistance to cleaning solutions as mentioned above. Unfortunately, conventional inner tubes made of quartz are insufficient in adhesion to CVD deposit and corrosion resistance to cleaning solutions.
Under these circumstances, the present inventors previously developed a new inner tube of glass-like carbon for CVD apparatus which prevents the occurrence of particles, resists corrosion by cleaning solutions, and meets other requirements mentioned above, and they filed an application for patent (Japanese Patent Laid-open No. 3504/2001).
There is another known way to improve adhesion to CVD deposit for components of glass-like carbon or components of graphite coated with glass-like carbon. It achieves its object by blast finishing which roughens the surface of the component. (Japanese Patent Laid-open No. 342068/2001 and Japanese Patent Publication No. 86662/1994)
It is also known that the inner tube and other components for CVD apparatus may contain impurities to contaminate wafers during CVD process and thus induced contamination causes resulting devices to malfunction. Therefore, the concentration of metal impurities in the surface of components should be no more than 10×1010 atoms/cm2, preferably no more than 5×1010 atoms/cm2. In fact, a value lower than 5×1010 atoms/cm2 has been achieved in the case of a commercial dummy wafer made of glass-like carbon prepared under adequately controlled conditions. This dummy wafer has a mirror-finished surface with a surface roughness no higher than 0.1 μm. As with this dummy wafer, the inner tube of glass-like carbon for CVD apparatus is required to have a surface metal concentration no higher than 5×1010 atoms/cm2.
Incidentally, the surface metal concentration is determined in the same way as used for determination of metal on the surface. of silicon wafers. The procedure starts with extracting metal from the sample with a mixed solution of 2% hydrofluoric acid and 2% hydrogen peroxide. Then the extract is analyzed by ICP-MS (inductively-coupled plasma mass spectrometry). The amount of metal thus determined is expressed in terms of the number of atoms per unit area of the sample used for extraction (atoms/cm2).
By the way, a tube of glass-like carbon is obtained by preparing a resin tube and then heating it in an inert atmosphere. The heating temperature is usually higher than 800° C., preferably 1000-1200° C., more preferably 1300-2500° C. During heating for carbonization, the resin tube greatly shrinks (about 20% by volume). This shrinkage makes it difficult to obtain a completely round tube of glass-like carbon.
There was proposed a method of achieving roundness by inserting a cylindrical graphite core in a resin tube before carbonization or in a glass-like carbon tube before heat treatment at high temperatures, as disclosed in Japanese Patent Laid-open No. 189470/1999 and Japanese Patent Publication No. 189471/199. The core has an outside diameter equal to the inside diameter which the glass-like carbon tube would have as the result of the resin tube shrinking after carbonization.
However, it turned out the above-mentioned conventional method that utilizes a cylindrical core does not always give a round glass-like carbon tube.